We evaluated two methods of estimating low-density lipoprotein cholesterol (LDL-C) in human sera: a recently reformulated "direct" method (Magnetic LDL-C, Polymedco) and a new, calculation-based indirect method. The new indirect method is based on the use of total cholesterol, triglycerides, and apolipoprotein B and has been claimed to be superior over the Friedewald formula for the estimation of LDL-C in non-chylomicronemic human sera. Both of these new methods were compared with two well- established "direct" LDL-C techniques (Sigma Direct LDL-C and Helena LipidProflle) and with conventionally calculated LDL-C (the Friedewald formula), based on the measurement of total cholesterol, triglycerides, and high-density lipoprotein cholesterol (HDL-C). In the Sigma Direct LDL- C method, HDL and very low-density lipoprotein (VLDL) are removed by immuno-absorption and the remaIning (total) cholesterol is measured as being equal to LDL-C in the absorbed serum. The Polymedco method removes LDL and calculates LDL-C as the difference between total cholesterol and the cholesterol content of the remaining lipoproteins. The Helena LipidProfile cholesterol method involves quantitative assessment of cholesterol by a coupled enzyme reaction following electrophoretic separation of various lipoprotein fractions in agarose gel. Hence, this method concomitantly measures cholesterol in all major lipoprotein fractions such as HDL, VLDL, LDL and chylomicrons. In addition to being measured with lipoprotein cholesterol electrophoresis, HDL-C was also assessed by a solid-phase dextran sulfate-Mg2+ method (Polymedco) in which non-HDL lipoproteins are magnetically removed by dextran sulfate-coated particles from the serum. The re-formulated Polymedco LDL-C method, which has been used to analyze over 1(x) normal and abnormal human sera, showed improved analytical performance over the earlier version and compared well with the Sigma Direct method in nonturbid sera. On the other hand, the new formula showed no clear advantage over the traditional Friedewald formula for calculation of LDL-C. There has been growing interest in the possible pathogenetic role and diagnostic importance of circulating anti-oxidized LDL auto-antibodies in atherosclerosis. In collaborative studies, we studied the analytical and diagnostic performance of an in-house and a commercial enzyme-linked immunosorbent assay (ELlSA) for the quantification of these antibodies in the sera of various groups of human subjects. Only the serum anti-oxidized LDL autoantibody titer obtained with the in-house ELlSA appeared to correlate with the development of atherosclerosis. The lack of similar correlations with the commercial assay and the lack of correlation between the two ELISA methods indicate that diagnostically important differences may exist among various assays for the measurement of these auto-antibodies.